Screen filters, and other sheet-based filters, are simple and economical and are thus widely used for separation of suspended particles from a fluid. Screen filters may be made of metal, plastic or synthetic cloths, secured to a substantially rigid skeleton element receivable within the filter housing.
Filtering screens are classified according to their filtration degree, defined by the diameter of the largest sphere that may pass the screen. A different indirect way to define a filtration degree is by the number of wires per inch or per cm (in case of a woven screen) referred to as ‘mesh’.
Where fine filtration is required, it is common to use backup, coarser filtering sheets, to support the fine screen. Sometimes these backup layers are used to perform pre-filtration to reduce the particle load on the fine screen. On different occasions these coarse layers are used to enable filtrate flow between conjugate layers parallel to the layers. The last filtration stage is always the fine screen filter layer. In addition, one or more layers may serve as spacers between neighboring layers to enable fluid flow outside the element.
In case of a metal screen and a metal cage, attachment therebetween may be facilitated by welding or otherwise fastening. However attaching the screen sheet to the filter element cage (skeleton) where the cage is made of plastic material or coated by plastic material is by applying heat to the plastic material to thereby melt its surface engaging the screen resulting in condensation of melted plastic material into the openings of the screen whereupon the screen attaches to the filter element skeleton.
However, the above method may be carried out only in case of a screen made of heat weldable material which can be plastic welded to the filter element cage/skeleton, or where the openings of the screen are sufficiently large to facilitate molten plastic flow into the openings, to thereby engage to the skeleton of the filter element.